Dichroic dyes are useful in guest-host liquid crystal devices, such as displays. Guest-host compositions are formed by dissolving dichroic dyes in a host solvent comprised of one or more liquid crystals. Typically, the guest-host composition is placed in a cell comprising two glass substrates coated with transparent electrodes and other compositions (e.g., alignments layer(s)) and having a adhesive layer perimeter as shown in FIGS. 11 and 12 (see the discussion of FIGS. 11 and 12, below). The liquid crystal and dye molecules of the guest-host composition are generally aligned predominantly parallel to the glass substrates. When aligned in this fashion, upon application of an electric field these molecules realign predominantly perpendicular to the glass substrates. The change in orientation results in a significant change in the intensity of absorption of the dye(s). This change in absorption is easily detectable to the eye.
Control of orientation is essential for efficient performance of such guest-host mixtures and the liquid crystal displays and other devices in which they are used. The order parameter, S, is a measure of the ordering of the dye in a liquid crystal host matrix (the method of measurement is discussed below). S ranges from 0 through 1. The higher the number, the higher the ordering of the dichroic dye in the host and the higher the contrast of a guest-host mixture containing the dye.
Several patents describe the benefit of having specific groups attached to dye chromophores to improve the dichroism, order parameters, and/or solubility of a dye in a liquid crystal host solvent.
U.S. Pat. No. 4,402,854 discloses 2,6-di[4-n-pentylcyclohexylbenzoyloxy]-anthraquinones. The liquid crystal moiety is bonded directly to the anthraquinone radical through an ester linkage but without a flexible alkylene spacer.
U.S. Pat. No. 4,676,923 discloses dichroic dyestuffs with phenylcyclohexyl and biphenyl moieties directly bonded to azo chromophores. No spacer group is used.
U.S. Pat. No. 4,588,517 discloses, among other things, liquid crystal moieties, such as biphenyl, cyclohexyl-cyclohexyl and phenylcyclohexyl moieties bonded to azo dyes through linkages having two linearly linked atoms, such as a ##STR1##
These patents suggest that bonding of such dye moieties to phenyl cyclohexane, biphenyl or cyclohexylcyclhexyl ring moities may improve the order parameter of guest-host systems and, thus, the optical properties of liquid crystal displays containing such guest-host systems. The compounds described in these patents have no more than two atoms linking the dye to the dicyclic ring system and, thus, are believed to be field and have low solubility in liquid crystal hosts.
The inventors have discovered that coupling of liquid crystal moieties to dichroic dyes moieties through flexible spacers can substantially improve the order parameter while still maintaining (and in some cases improving) solubility of the dichroic dyes in liquid crystal host solvents without changing the fundamental absorption characteristics of the dye chromophore.